(1) Field of the Invention
The present invention is directed to the improved production of activated carbon. More particularly, it is directed to the production of highly microporous activated carbon. Specifically, the present invention is directed to the production of highly microporous activated carbon from an activated carbon precursor by a subsequent chemical activation process employing potassium hydroxide. One use of such activated carbon is in the adsorption of gaseous hydrocarbon fuels, including natural gas.
(2) Description of the Prior Art
Practical storage of gaseous hydrocarbon fuels, such as natural gas which is comprised primarily of methane, for such uses as a vehicle fuel and the like involves portable containerization of the gas. Natural gas, in particular, is a leading contender for use as an alternative fuel for automobiles, particularly in areas designated as "zero emission" zones under the 1990 Clean Air Act. The majority of natural gas vehicles operating in the United States use compressed natural gas at pressures of up to 3600 psi. However, low pressure storage systems are being developed in which natural gas is contained in storage containers packed with adsorbent material to achieve near-liquid methane density. The storage containers may be formable or nonformable tanks, cylinders, or other closed vessels. Economic evaluations by the natural gas industry indicate that adsorbed natural gas (ANG) would be comparable economically with compressed natural gas (CNG) at a deliverable gas capacity of 150 volumes of gas per container (cylinder) volume (v/v) at a storage pressure of 500 psi (measured at 25.degree. C.).
Natural gas, which is primarily methane, is adsorbed in pores and on surfaces of the adsorbent medium. Under such conditions, the adsorbed gas assumes properties not unlike those of its liquid state. Typical adsorbents are solids with pores and fissures throughout their structure. Methane molecules preferentially adsorb in pores having a diameter of about 10-15 Angstroms (.ANG.). The carbon material of the present invention may also be suitable for storage of other gases of a similar molecular size.
Active carbon long has been used for removal of impurities and recovery of useful substances from liquids and gases because of its high adsorptive capacity. Generally, "activation" refers to any of the various processes by which the pore structure is enhanced. Typical commercial activated carbon products exhibit a surface area (as measured by nitrogen adsorption as used in the B.E.T. model) of at least 300 m.sup.2 /g. For the purposes of this disclosure, the terms "active carbon" and "activated carbon" are used interchangeably. Typical activation processes involve treatment of carbon sources--such as resin wastes, coal, coal coke, petroleum coke, lignites, polymeric materials, and lignocellulosic materials including pulp and paper, residues from pulp production, wood (like wood chips, sawdust, and wood flour), nut shell (like almond shell and coconut shell), kernel, and fruit pits (like olive and cherry stones)--either thermally (with an oxidizing gas) or chemically (usually with phosphoric acid or metal salts). Such activated carbons maintain the original macrostructure of the starting material and, therefore, a similar pore distribution of micropores of less than 20.ANG. in width, mesopores of 20.ANG. (divided between small mesopores of 20.ANG. to less than 50.ANG. in width and large mesopores of 50.ANG. to 500.ANG. in width), and macropores of greater than 500.ANG. in width.
As the surface area of an active carbon is directly proportional to the carbon's microporosity and since the methane adsorption capacity of an active carbon is enhanced by increasing its volume of micropores (less than 20.ANG. in width) and small mesopores (20-50.ANG. in width), as a percentage of total pore volume, activation methods are sought which are pore size specific. In particular, micropores in the range of from above 8.ANG. to about 20.ANG. are suitable for adsorption of methane. More particularly, pore sizes of from about 10.ANG. to about 20.ANG. in width are preferred for methane adsorption. Most preferred are pore sizes of from about 10.ANG. to about 15.ANG.. Therefore, carbon materials are desirable which are comprised of a high volume (e.g., greater than 50%) of pores less than 16.ANG. in width as a percentage of total pore volume. Such materials which are comprised of a higher volume (e.g., greater than 80%) of pores less than 20.ANG. in width as a percentage of total pore volume also are desirable. Also desirable are such materials comprised of an extremely high volume (e.g., greater than 95%) of pores less than 50.ANG. in width as a percentage of total pore volume.
Citing disclosures of potassium hydroxide (KOH) activation of coal in U.S. Pat. Nos. 3,764,561 and 4,082,694, the patentees of U.S. Pat. No. 4,769,359 teach the production of active carbon which enables high adsorption of gases per unit volume by treating coal with a liquid mixture comprising KOH and sodium hydroxide (NaOH) and subsequently carbonizing at 500.degree.-800.degree. C. A method of producing activated carbon with a large surface area and a low sulfur content also is taught in U.S. Pat. No. 5,064,805 by mixing coconut shell char with melted potassium hydroxide hydrate at a temperature sufficiently high to cause activation. Also, U.S. Pat. No. 4,082,694 teaches solid KOH activation of specific carbonaceous feeds including coal, coal coke, and petroleum coke to produce cage-like microporous structures particularly useful for water purification.
Chemical activation of wood-based carbon with phosphoric acid (H.sub.3 PO.sub.4) is disclosed in US. Pat. No. Re. 31,093 to improve the carbon's decolorizing and gas adsorbing abilities. Also, U.S. Pat. No. 5,162,286 teaches phosphoric acid activation of wood-based material which is particularly dense and which contains a relatively high (30%) lignin content, such as nut shell, fruit stone, and kernel. Zinc chloride (ZnCl.sub.2) also is a common chemical activation agent. Phosphoric acid activation of lignocellulose material also is taught in U.S. Pat. No. 5,204,310 as a step in preparing carbons of high activity and high density.
Also, U.S. Pat. No. 4,769,359 teaches producing active carbon by treating coal cokes and chars, brown coals or lignites with a mixture of NaOH and KOH and heating to at least 500.degree. C. in and inert atmosphere. U.S Pat. No.5,102,855 discloses making high surface area activated carbon by treating newspapers and cotton linters with phosphoric acid or ammonium phosphate. Coal-type pitch is used as a precursor to prepare active carbon by treating with NaOH and/or KOH in U.S. Pat. No. 5,143,889. Finally, U.S. Pat. No. 5,292,706 teaches storing natural gas under pressures of 1400 to 4500 kPa using a carbon sieve adsorbent made by impregnating a polymer precursor of the carbon molecular sieve with additional monomers and polymerizing the monomers before carbonizing the precursor.
None of these activated carbons, however, achieve the desired objective of providing 150 v/v of deliverable gas capacity at 500 psi. Such a carbon is taught, in U.S. patent application Ser. No. 08/143,551, (U.S. Pat. No. 5,416,056) to be produced by a two-step chemical activation process. A lignocellulosic material was first activated with phosphoric acid and then activated with potassium hydroxide under thermal conditions. While permitting small scale, laboratory production of the desired carbon material, the disclosed process has several disadvantages which preclude commercial production to avail the product to public use. The large volume of wet carbon material, upon blending with activating agent solution, presents material handling problems. Also, the batch nature of the process, combined with the requirement of a relatively long duration of thermal treatment with gradual incremental temperature increases, precludes efficient, economical production rates. In addition, it was discovered that the disclosed process leaves residual potassium which, along with calcium adsorbed from the wash water, adversely impacted the highly microporous carbon's performance.
Therefore, the objective of this invention is to provide an improved process for manufacturing a highly microporous activated carbon capable of meeting the industry target for a deliverable capacity of a gaseous hydrocarbon fuel stored on activated carbon. It is also an objective of this invention to provide a highly microporous active carbon material that is specific for storage of methane in natural gas to provide a deliverable capacity of 150 v/v of the methane at 500 psi (at 25.degree. C.). It is a further objective of this invention to provide a method for storing natural gas at low pressure using the highly microporous activated carbon.